1. Technical Field
The invention relates to an audio mixer system for generating coded audio data with a sound source at a desired location.
2. Description of the Related Art
Quadrophonic audio systems are known which enable four audio input signals to be matrix encoded into two conventional stereo channels which are then decoded back to four audio output signals for playback.
In one well known such system, which has been commercialized as the Dolby Pro Logic system (Dolby and Pro Logic are trademarks of Dolby Laboratories Inc), the four channels correspond to the conventional stereo speakers placed at the front left and right corners of a room, together with a speaker placed at the center of the front stage and one or more surround speakers placed generally to the rear of the listener. These four channels are encoded into a stereo data stream as follows. The signals for the left and right speakers are placed in conventional fashion into the left and right stereo channels. The center signal is encoded in equal amounts into both left and right channels. The surround signal is also encoded in equal amounts into both left and right channels, but with a 180 degree phase shift between the information being encoded into the two channels, so that the surround signal can be recovered from the difference between them.
For such systems active decoding technology has become available which enables the four channels to be recovered with acceptable levels of crosstalk between the channels to generate an illusion of a 360 degree sound field.
Whilst static mixing of audio signals from sound sources at different locations into an encoded data stream of this type is relatively simple, a problem arises if it is desired to allow any particular sound source to be moved around in real time within the 360 degree field. This is because whenever the sound source crosses the nominal location of the surround speaker there is a phase discontinuity between the signals generated with the sound source just to either side of the surround speaker position. This discontinuity can result in an audible click, which is annoying for the user.
It is an object of the present invention to provide an audio mixer for generating coded data having a sound source at a desired and controllable location and which does not suffer from the above artifact.
In brief, this is achieved by an audio mixer system for producing coded output in which at least a left audio signal, a right audio signal and a surround audio signal are encoded in two output channels so that the surround signal can be decoded from the difference of the two output channels. The system comprises means for generating position data designating a desired position for a sound source in a 360 degree sound field. Logic is provided for determining the relative volume of the sound source in the left, right and surround audio signals from the position data. A signed continuity factor is maintained so that the sign of the continuity factor is changed in response the desired position crossing a nominal position of the surround signal in the sound field and logic is provided for encoding the sound source data into the two output channels in accordance with the determined relative volume of the sound source in at least two of the left, right and surround signals each multiplied by the continuity factor.
In a preferred embodiment, user input means are provided for allowing a user to designate the desired position. Preferably, the user input means comprises means to generate a window on a display screen and pointer means to manipulate an icon within the window to indicate a desired position for the sound source. The icons can be in the form of images representing musical instruments associated with the sound source.